The requirements for transport network resilience are quite stringent. They include 99.999% availability, two hour repair windows, and 50 millisecond (ms) protection switching times. The 50 ms protection switching time is actually one portion of the generally understood maximum 60 ms interruption, where up to 10 ms are allocated to fault detection and 50 ms are allocated to performing the protection switch. The remainder of this discussion refers only to the 50 ms portion.
The 50 ms requirement dates back to the early days of computer networks and is now an expected attribute of transport equipment. While this requirement originally applied to redundancy switching in equipment, such as stand-alone M13 multiplexers that takes 28 DS1 inputs and combines them into a single 44.736 Mbps DS3 stream, it is now the basis for several industry equipment specifications, including GR-499-CORE (GR-499-CORE, Telcordia “Transport System Generic Requirements (TSGR): Common Requirements,” Issue 2, December 1998), and various SONET network protection schemes, including ANSI T105.01 (ANSI T1.105.02, Alliance for Telecommunications Industry Solutions, “Synchronous Optical Network (SONET)-Automatic Protection,” 2000), Telcordia GR-253-CORE (GR-253-CORE, Telcordia, “Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria,” Issue No. 3, September 2000), GR-1230-CORE (GR-1230-CORE, Telcordia, “SONET Bidirectional Line Switched Ring Equipment Generic Criteria,” Issue No. 4, December 1998), and GR-1400-CORE (GR-1400-CORE, Telcordia, “SONET Dual-Fed Unidirectional Path Switched Ring (UPSR) Equipment Generic Criteria,” Issue No. 2, January 1999).
The drivers that led to such a stringent requirement generally include the signal timing of analog trunks, timeouts of automatic teller machines, Systems Network Architecture (SNA) links, and Signaling System No. 7 (SS7) links. Regardless, the 50 ms protection switching is an expected part of today's transport network. In fact, it is often a criteria of transport network service level agreements and is often included in published tariffs of local exchange carriers.
There has been some discussion about relaxing this requirement, particularly as less sensitive data services proliferate. Higher layer data services typically provide network resiliency with a longer time frame on the order of seconds for the recovery of traffic. This is at least partially based on the premise that the transport network will provide consistent and fast 50 ms protection switching times. If the protection switch times of the transport network were to lengthen and become less consistent, an additional protection time frame would be introduced into the network. These additional protection times add layers of complexity that the carriers would need to manage, while sharing many of the same transport resources. An overlay approach could be considered, but it brings with it a whole host of business, operational, and technical issues. Given that many end-user services rely explicitly or implicitly on this requirement, the 50 ms protection switch time is here to stay.